Delta utilization by salmon
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River Deltas provide support for juvenile salmon during a critical phase in their life history, and use delta wetlands for refuge, forage, and smoltification. Increasing the ability of Deltas to support threatened Chinook and chum salmon outmigration is a primary motivating factor for restoration and protection activity.
The following pages are associated with Delta sediment dynamics and vegetation:
All pacific salmon use river deltas during transition from freshwater fry to saltwater smolts. Each species uses nearshore habitat differently, with Chinook and chum having the most extensive use (Fresh 2006; Simenstad et al 1982). Within Chinook, there is substantial variation in fish arrive in the delta, and how long they stay (Beamer et al 2005). Although these differences likely exist within chum populations, they have not been well described. Fish reared in hatcheries likely differ in how they use the delta and nearshore (Fresh 1997), and since we have only recently been able to discriminate between hatchery and wild fish in our studies, it is unclear how our current understanding may be biased by the prevelance of hatchery fish in many systems.
Chinook fry and parr enter deltas from December through September, with the largest migrations typically from May to July, as parr, although this varies by population. Some of the small fry leave the delta very quickly and appear to preferentially use barrier lagoons and barrier estuaries in the nearshore (Beamer et al 2005). Some early arrivals may stay as long as 120 days in the delta (Simenstad et al 1982; Beamer et al 2005). Larger fish that arrive later (Healy 1980; Healy 1982), don't stay as long as those that arrive early as fry. Juveniles may increasingly use larger channels orleave delta altogether as shallow water temperatures increase in summer.
Chum fry enter the delta shortly after emergence from eggs beginning with Summer run fish emergence in December (Salo 1991), Chum migrants appear to distribute broadly both in natal estuaries, as well as in a range of estuarine habitats in the nearshore (see Fresh 2006).
Current research considers habitat patches in the context of their landscape, such that configuration and composition of habitats and their relationship to rivers and currents are likely to affect realized habitat services (Fresh 2006). Simenstad and Cordell 2000 describe two attributes:
- Opportunity - the the degree to which a given habitat can be accessed by juvenile fish, and
- Capacity - the ability of the habitat to provide services like forage and refuge, once accessed.
Fresh 2006 provides a range of attributes anticipated to affect opportunity and capacity. Juvenile salmon feed throughout their migration on a wide range of prey based both on availability and preference. Juvenile salmon prey appear to be largely dependant on detrital food webs supported by delta vegetation.
Our understanding of delta use by juvenile salmon is strongest for Chinook, and based on extensive sampling in the Skagit Delta. Beamer et al 2005 both identified the use of small dendritic channels accross the entire delta landscape, while also providing evidence that tidal channel networks closest to main migratory routes have a higher density of juvenile Chinook salmon. Since small tidal channels dewater between tides, low tide refuge may also be important to habitat utilization.
Fresh 2006 provides a broad assessment of how different human actions which alter the landscape are likely to in turn affect the opportunity or capacity for salmon to find habitat services.
Methods and Issues
Simenstad & Cordell 2000 propose a conceptual framework for evaluating how restoration of salmonid habitat in estuaries increases survivability by characterizing and relating capacity, opportunity, and realized function. They suggest that we have limited data to connect structural metrics like capacity and opportunity to realized functions such as growth, consumption rate, or survival, and that while such an assessment approach addresses local effects of habitat of fish, it fails to address how the occupation of landscapes by diverse life histories affects the viability of populations.
There are four different methods in common use for estimating fish density:
- Beach Seine
- Fyke Net
- Screw Trap
- Minnow Traps
There are a wide range of issues to consider in Fish Sampling Design both at the scale of the site and techniques used, as well as how sampling is used to describe variation of salmon use over the delta, over time, and between years.
Common approaches for demonstrating retoration of opportunity within an action is to focus on one or more target salmon species at the juvenile life stage and determine if juvenile salmon use new channels and marsh habitat at densities similar to a reference site (adjusted for channel allometry differences) and as new tidal channel systems develop whether juvenile salmon use increases proportionally. This can be in combination with other measures of juvenile salmon habitat including their macroinvertebrate prey base. Juvenile salmon densities are often used as a good proxy for habitat use by salmon. These are usually compared to reference sites. Diagnostic monitoring by contrast attempts to explain the physical and biological factors that are controlling the change in juvenile salmon density (Rice et al 2001).
Site conditions may constrain whether a fyke net, beach seine or both are used to sample juvenile salmon. Beyond site and system scale monitoring, there is a desire to link site and system scale changes in juvenile salmon densities with recovery targets for juvenile salmon at various scales. This requires an understanding of landscape variables that affect status and trends of juvenile salmon densities that can be independent of river delta restoration actions. These are numerous and can reduce the inference limits of site and system scale monitoring. Standardizing methods of experimental design and analysis can go a long way in supporting meta-analysis of juvenile salmon density trends and how juvenile density factors into regional population level recovery.